Solenoid actuated three-way valve

ABSTRACT

A solenoid actuated three-way valve is formed with a central passage communicating at one end with a vent passage via a first valve seat and communicating at its opposite end with a pressure supply port via a second valve seat. A control port directly communicates with the central passage at all times. An armature is slidable within the passsage to block one of the valve seats while opening the other and is resiliently biased toward one seat by a compression spring and magnetically biased toward the other seat by energization of the solenoid coil. Flux washers at opposite ends of the coil contact a ferromagnetic casing to provide an efficient flux path enabling rapid response of the armature to coil energization. One valve seat is loccated at one end of a pole piece threadably adjustable within one of the flux washers to establish a working gap in the magnetic circuit of a minimum width of the valve. A relatively large return gap is provided to minimize frictional losses within the valve.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my co-pending applicationSer. No. 07/282,587, filed Dec. 12, 1988, now abandoned.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention is directed to a solenoid actuated three-way valveof the type employed in pulse width modulated pressure control systemswherein the pressure at a control port of the valve is regulated bycyclically connecting the control port alternately to a high-pressuresource and a low-pressure source to achieve a pressure at the controlport proportional to the percentage of time during which the controlport is connected to the high-pressure port.

II. Description of the Related Art

Valves of the foregoing type are being increasingly employed to controlautomotive transmission systems by regulating the engagement pressure ofvarious clutches within the transmission to regulate the torquetransmitted through the individual clutch. An on-board microprocessorreceives various inputs representative of vehicle operating conditions,such as vehicle speed, engine RPM, throttle setting, etc. The processoris programmed to compute the optimum transmission ratio in accordancewith the various inputs. In response to the inputs, the processorgenerates a pulse width modulated control signal at a constantfrequency, typically in the order of 60 Hz, which controls energizationof the solenoids of the various solenoid valves. During each cycle ofthe pulse width modulated control signal, the coil of a solenoidactuated valve is energized for a predetermined percentage of the cycleperiod and deenergized for the remainder of the cycle. Over a series ofsuccessive cycles, the pressure at the control port of the valve,assuming the pressure at the low-pressure port is zero, will be apercentage of the pressure at the high-pressure port equal to thepercentage of time the high-pressure port is connected to the controlport.

The requirements of a solenoid valve employed in such a system arebasically that shifting of the valve armature between its alternatepositions must closely and accurately track the rapidly repeatedenergization and deenergization of the solenoid coil. This requires thedevelopment of a maximum axial force applied to the solenoid armatureupon energization of the coil, and minimum parasitic losses, such asfriction, eddy currents, etc. within the assembly.

The present invention is directed to a solenoid valve which efficientlymeets these last requirements, as well as the universal requirement ofthe automotive industry of low unit cost and simplified construction.

SUMMARY OF THE INVENTION

A solenoid valve embodying the present invention includes a one-piecebobbin of a molded, non-magnetic material formed with a central passageextending axially through the bobbin. At one end of the bobbin, a pairof projecting posts are integrally formed to serve as mounts forelectrical connectors to be connected to the opposite ends of thewinding of the solenoid coil which is would upon the bobbin. Annularflux washers are mounted on the bobbin adjacent each end of the bobbin,one of the flux washers having a pair of bores for passing the connectormounting posts on the bobbin and a central threaded bore into which apole piece is adjustably threaded with the inner end of the pole pieceblocking one end if the central passage through the bobbin. The polepiece is formed with an axial passage which extends from a valve seat atthe end disclosed within the central passage to a vent port opening toatmosphere. The other flux washer is sandwiched between the opposite endof the bobbin and a valve housing with its inner periphery at a fairlysubstantial radial spacing outwardly from their central passage. Thevalve housing has a chamber communicating with the central passage inthe bobbin. This chamber is at all times in communication with a controlport via a passage in the valve housing. A supply port in the valvehousing communicates via a supply passage in the housing with a valveseat opening into the chamber in coaxial alignment with the centralpassage. An armature is slidably received within the central passagebetween the two valve seats and is axially movable within the passagebetween end limits defined by the engagement of the armature with one orthe other of the two valve seats. The armature is grooved along itsouter surface to provide a substantially unrestricted flow passagebetween the opposite ends of the armature while enabling the armature toslidably engage the central passage wall to maintain the armatureaccurately centered within the passage.

In a preferred form of the invention, a compression spring is engagedbetween the pole piece and armature to bias the armature to seal thevalve port which communicates with the supply port, the string biasexceeding the pressure exerted on the armature by pressure at the supplyport. In this arrangement, the control port is thus normally in directfluid communication with the vent port. When the solenoid is energized,the armature is magnetically shifted to unseat the armature from thevalve seat connected to the supply port and to simultaneously seal thevalve port in communication with the vent passage, the magnetic force ofthe energized coil being augmented by the supply port pressure. In thisenergized condition of the solenoid, the control port is connected tothe supply port.

The flux washers, bobbin and valve housing are held in assembledrelationship with each other by a sheet metal housing which axiallyclamps the parts into assembled relationship with each other and alsoserves as a portion of the path for the magnetic flux to increase theefficiency of the magnetic shifting of the armature. A relatively shortaxial air gap between the pole piece and armature maximizes the magneticshifting force which a relatively large radial air gap between theopposite end of the armature and flux washer minimizes frictionproducing radial forces on the armature.

Other objects and features of the invention will become apparent byreference to the following specification and to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single figure of drawings shows a cross-sectional view, taken on anaxial plane, of a three-way solenoid actuated valve embodying thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The valve shown in the drawing includes a solenoid bobbin designatedgenerally 10 which may be conveniently molded from any of severalsuitable non-magnetic, thermoplastic materials. The bobbin is formedwith an annular recess 12 in which the solenoid coil S is wound and theends of the wire of the coil are led through bores in the bobbin (notshown) to electrical connectors 14 fixedly mounted in a pair of axialprojections 16 formed at one end of the bobbin.

The bobbin is also formed with a central passage 18 which extendsaxially entirely through the bobbin.

At the upper end of bobbin 10, an annular flux washer 20 offerromagnetic material is seated upon a shoulder 22 formed on bobbin 10.Flux washer 20 is formed with a pair of bores 23 located to looselyreceive connector projections 16. A central bore 24 through washer 20 iscoaxially aligned with central passage 18 of the bobbin and isinternally threaded to threadably receive a pole piece 26 which islikewise formed of a ferromagnetic material.

The lower end of pole piece 26 projects into the upper end of centralpassage 18 and is sealed to the inner wall of passage 18 as by an O-ring28 which will accommodate axial adjustment of the pole piece relative toflux washer 20 for purposes referred to below. A passage 30 extendsaxially through pole piece 26 to open at its lower end into centralpassage 18 via an annular valve seat 32 formed on the lower end of thepole piece. In the particular embodiment shown in the drawing, passage30 opens at its upper end to atmosphere and passage 30 constitutes avent passage.

For reasons to be discussed below, pole piece 26 is threadablyadjustable in washer 20 axially of passage 18 to establish an accuratelydetermined working gap or axial spacing between the upper end of thesolenoid armature 62 and valve seat 32 when the armature is in theposition shown in the drawings.

A second flux washer 34 is mounted at the lower end of bobbin 10 andformed with a central bore 36 received upon a reduced diameter axialextension 38 formed on the lower end of the bobbin. For reasons to bediscussed below, the internal diameter of bore 36 is between 110% and140% of the diameter of passage 18 in the bobbin.

Underlying flux washer 34 is a valve housing designated generally 40.Housing 40 is formed with a central recess 42 extending downwardly fromits upper end which is dimensioned to slidably receive the lower end ofprojection 38 of the bobbin A central bore 44 extends upwardly throughhousing 40 from a supply port 46 located at the lower end of housing 40.Passage 44 opens into chamber 42 via a valve seat 48 located at thebottom of recess 42. At diametrically opposite sides of passage 44, apair of bores 50 extend downwardly through valve housing 40 from itsupper end to open through an undercut shoulder 52 on housing 40 throughcontrol ports 54. Valve housing 40 is conformed to be axially insertedinto a manifold, not shown, and sealed to the manifold as by O-rings 56,58.

Valve housing 40, bobbin 10 and flux washers 20 and 34 are fixedlysecured in the assembled relationship shown in the drawing by a sheetmetal casing 60 which firmly clamps these parts in axial, face-to-facerelationship with each other. Casing 60 is of a ferromagnetic materialand also functions as a part of the flux path of the magnetic fluxinduced by energization of solenoid coil S.

Within central passage 18, an armature 62 is slidably mounted for axialmovement within passage 18 between valve seats 32 and 48. For reasons tobe discussed below, the outer diameter of armature 62 is such as toestablishing a close, but freely sliding fit within passage 18. Theaxial length of armature 62 is less than the spacing between the twovalve seats (as adjusted by threadably positioning pole piece 26 in fluxwasher 20) so that when one of the heads 64, 66 is seated, sealedengagement with its valve seat, the other valve head is disengaged fromits associated seat by a distance sufficient to accommodate an adequateflow of fluid between the head and seat. Axial grooves 68 in the outersurface of armature 62 provide a substantially unrestricted flow pathbetween the opposite ends of the armature to equalize fluid pressure atopposite ends of the armature.

Spring 70 resiliently biases the armature 62 downwardly as viewed is thedrawing to place valve head 66 in seated engagement upon valve seat 48,there by blocking fluid communication between supply port 46 and chamber42. Control ports 64 are in fluid communication at all times with thechamber 42 via bores 50. With armature 62 in the position shown in thedrawing, chamber 42 is in fluid communication with vent passage 30 viavalve seat 32 and grooves 68 in armature 62.

Electrical energization of solenoid coil S is operable to generate amagnetic flux which magnetically biases armature 62 upwardly from theposition shown in the drawing until the valve head 64 at the upper endof armature 62 contacts valve seat 32. With valve seat 32 closed byvalve head 64, communication between vent passage 30 and control ports54 is blocked, but at the same time valve head 66 has been liftedupwardly out of engagement with valve seat 48 so that supply port 46(connected to a source of air under pressure, not shown) is in fluidcommunication with control port 54 via passage 44, valve seat 48 andbores 50.

The valve is especially intended for use in a system in which solenoidcoil S is cyclically energized and deenergized under the control of apulse width modulated control signal. In a general application, whereair is used as the pressure fluid, supply port 46 will be connected to asource of air under pressure and control ports 54 will be connected tothe actuating chamber of a pneumatically actuated device, while passage30 will be simply vented to atmosphere. The pulse width modulatedcontrol signal which controls energization of solenoid coil S cyclicallyenergizes and deenergizes the solenoid coil so that the period of timewithin a given cycle during which coil S is energized is varied inaccordance with variations of the control signal derived from amicroprocessor. As explained above, the microprocessor will receiveinputs representing various operating parameters and generate a controlsignal output in accordance with the processor program.

As explained above, when the solenoid is deenergized and the armature 62is in the position shown in the drawing, control ports 54 and anycontrolled device connected to it will be vented to atmosphere via pores50, chamber 42, grooves 68 in armature 62 and vent passage 30. Pressurefrom the supply source at supply port 46 is isolated from control port54 at this time because valve head 66 of the armature is seated uponvalve seat 48.

Upon energization of solenoid coil S, armature 62 is magnetically biasedupwardly to disengage valve head 66 from valve seat 48, thereby placingsupply port 46 in communication with control port 54 to supply pressurefrom the pressure source to the control device. Simultaneously, valvehead 64 engages valve seat 32 to isolate vent passage 30 from thecontrol and supply ports 54, 46.

When armature 62 is rapidly and continually cycled between its twopositions, control port 54 is alternately connected to vent passage 30and to supply port 46. When control port 54 is connected to the supplyport 46, the pressure at control port 54 will tend to increase toapproach the pressure of the source connected to supply port 46, whilewhen control port 54 is connected to vent via passage 30, the pressureat control port 54 will tend to drop toward zero or atmosphericpressure. By continuously applying alternate on-off cycles, the pressureat port 54 will stabilize at a pressure which is a percentage of thesupply source pressure equal to the percentage of time over the giventime period during which the solenoid coil was energized.

Typical operating frequencies of a pulse width modulating control systemare in the neighborhood of 60 Hz, which means that the armature 62 mustbe capable of rapid movement between its alternate positions. Movementof armature 62 to the normal (solenoid deenergized) position shown inthe drawing is essentially under the control of spring 70 and presentsno substantial design problems. Movement of the armature to its upperposition as viewed in the drawings depends upon the magnetic fluxdeveloped by energization of the solenoid, and in particular the axialflow of the magnetic flux across what will be referred to as the workinggap which is the axially spacing between valve head 64 and the opposedvalve seat 32. The development of a maximum axially directed magneticforce upon armature 62 for a given number of ampere turns of solenoidcoil S requires a consideration of several design parameters.

In addition to acting as a return spring, spring 70 also functions as apressure relief setting in the event the pressure at supply port 46should, for some reason or other, increase above a desired value.

In the valve configuration described above, the magnetic circuit or flowpath of magnetic flux induced by energization of solenoid S has two "airgaps", one of which is the working gap referred to above -- i.e. theaxial spacing between valve head 64 at the top of the armature and theopposed valve seat 32 on the pole piece. The second "air gap" is thatbetween the wall of bore 36 in the lower flux washer 34 and the outerdiameter of armature 62.

The flow of flux across the working gap between valve head 64 and itsassociated valve seat 32 is directed axially of the coil and it is themagnetic force developed across this gap which acts to shift armature 62upwardly against the action of spring 70 to seat valve head 64 againstseat 32. Because the magnetic force developed across the working gapvaries inversely with the square of the distance across the gap,obviously rapid response of the valve dictates this distance be as smallas possible. The minimum distance or working gap length is establishedby the minimum fluid flow requirements of the valve -- in other words,there must be enough space left between valve head 64 and valve seat 32when in the position shown in the drawing to accommodate adequate fluidflow through the opened valve 64, 32. This spacing, due to manufacturingtolerences and variations in material characteristics is difficult tocalculate, hence the threaded adjustment of pole piece 26 permits theworking gap to be adjusted after assembly.

Flow of flux across the return gap between the surface of bore 36 in thelower flux washer and the outer diameter of armature 62 is essentiallyflow along paths extending radially of the longitudinal axis of thearmature. This radially directed flow of flux has substantially nodirect effect upon axial movement of armature 62 beyond the fact thatthe power required to generate the flow of flux across the return gap isnot available to assist in driving the armature is axial movement. Thus,it is generally considered by the prior art to be good design practiceto make the return gap or radial clearance between the armature and bore36 as small as possible to minimize the power loss within the magneticcircuit. This reasoning, however, overlooks an important fact which hasbeen ignored in the prior art.

It is generally assumed that because the inner wall of bore 36 and theouter surface of armature 62 are circular and coaxial with each other,the magnetic forces induced by the flow of magnetic flux radially acrossthe return gap counterbalance each other and inherently result in anequilibrium condition in which there is no not force tending to move thearmature in any radial direction. While this assumption is theoreticallycorrect, this theoretical equilibrium of forces is based on an overlyoptimistic assumption that the opposed bore and armature surfaces areprecisely circular and precisely coaxial with each other and that themagnetic field is precisely symmetrical. In theory, such equilibrium ispossible, in practice it cannot be achieved in devices produced on amass production basis.

If this magnetic equilibrium is not achieved, a net force will beapplied biassing the armature in a direction radially of its axis andthis force will increase with the resultant movement of the armature.The armature will move radially until it is prevented from movingfurther by engagement with the wall of passage 18. This will result infrictional forces between the armature and passage wall which aregenerally proportional to the force which presses the armature againstthe wall. Where the return gap is minimized in accordance withconventional design practice, the forces urging the armature against thepassage wall will be relatively high, and the resultant frictionalforces can typically absorb 20% or more of the power required to axiallyshift the armature by energization of the solenoid coil. Power losses ofthis magnitude can equal or exceed any gain achieved by minimizing thereturn gap.

Thus, in accordance with the present invention a relatively wide returngap is employed by making the internal diameter of bore 36 a diameterwhich is somewhere within the range of 110% to 140% of the outsidediameter of the armature which, desirably is made as closely fitting aspossible to the internal diameter of passage 18 as is compatible with afreely sliding fit.

The magnetic reluctance of the return gap, where the gap is relativelylarge, as described above, may be reduced by increasing the area of thereturn gap -- i.e. increasing the axial thickness of flux washer 34. Asa rough rule of thumb, for a fast responding solenoid valve of the typeunder consideration the thickness of the flux return washer 34 should bebetween 30 and 50 percent of the axial length of solenoid coil S.

Because the unbalanced magnetic force across the return gap whichinduces radial movement of the armature increases rapidly as thearmature moves in response to this force, the normal radial clearancebetween the outer diameter of the armature and the wall of passage 18should be made as small as possible to minimize this movement, while atthe same time allowing the armature to slide freely through the passage.The axial grooves 68 in the armature enable a major portion of thearmature surface to slidably contact the passage wall, while at the sametime providing an adequate flow path for the working fluid to flowthrough passage 18 from one end of the armature to the other.

The ratio of the mechanical radial clearance between the armature andwall of passage 18 to the magnetic clearance (return gap) between theouter wall of the armature and bore 36 in flux washer 34 has asubstantial effect on the frictional forces discussed above. The ratioof the mechanical clearance to the magnetic clearance should be as lowas possible and should not exceed 0.2.

Selection of the material from which the armature, flux washers polepiece 26 and casing 60 are constructed depends on the particularapplication of the valve, taking into account cost considerations.

Where cost considerations are paramount, as is frequently the case inautomotive applications, the material having the standard designation12L14 (a medium carbon leaded steel) is a preferred material. Inaddition to its low material cost, 12L14 is easily machined, aconsideration where parts of complex state, such as the slotted armature62 and upper flux washer 20, are involved.

However, most automotive applications require the valve to operate attemperatures well below freezing where higher forces are required toovercome the viscosity of a cold working fluid controlled by the valve.For low temperature operation, a 2.5% silicon iron is preferred;however, this material is more costly than 12L14 and more difficult tomachine.

Where parts of complex shape are employed, a powdered metal of 0.45% to0.9% phosphorus iron alloy may be the preferred material because it maybe pressed and sintered to form parts of complex shape at a relativelylow manufacturing cost and has somewhat better performancecharacteristics than 12L14.

While one embodiment of the invention has been described in detai1, itwill be apparent to those skilled in the art the disclosed embodimentmay be modified. Therefore, the foregoing description is to beconsidered exemplary rather than limiting, and the true scope of theinvention is that defined in the following claims.

What is claimed is:
 1. In a solenoid actuated three way valve intendedfor rapidly repeated cyclic operation, said valve comprising anenergizable solenoid coil, an elongate armature mounted within a mainpassage extending coaxially through said coil and having first andsecond valve head means respectively located at its opposite ends, firstand second valve seat means at the respective opposite ends of said mainpassage, said first and second seat means be spaced from each otheraxially of said main passage by a distance greater than the length ofsaid armature, means defining first and second flow passages openinginto said main passage through the respective first and second valveseats, spring means biassing said armature in one direction to normallymaintain said first valve head means engaged with said first valve seatto block fluid communication between said main passage and said firstpassage, said solenoid coil being operable when energized tomagnetically bias said arpature in the opposite direction to engage saidsecond valve head means with said second valve seat to block fluidcommunication between said main passage and said second passage, andmeans defining a third flow passage opening into said main passage,theimprovement wherein said valve includes first and second annular fluxwashers located at the respective opposite ends of said coil, a hollowgenerally cylindrical metal outer housing member externally surroundingsaid coil and contacting the outer periphery of said first and secondflux washers to define a low reluctance magnetic flux path therebetween,said first valve seat being located axially beyond that side of saidfirst flux washer remote from said coil, a pole piece mounted upon saidsecond flux washer and projecting from said second flux washer into saidpassage beyond the adjacent end of said coil, said second valve seatbeing located on said pole piece and said pole piece being adjustableaxially of said main passage to adjustably establish a minimum workinggap adequate to accomodate fluid flow between said second valve seat andsaid second valve head means when said solenoid coil is deenergized,said first flux washer having a central bore therethrough coaxial ofsaid main passage and of a diameter falling within the range of 110% and140% of the diameter of said main passage.
 2. The invention defined inclaim 1 wherein said main passage is defined at least in part by acentral bore extending through a solenoid coil supporting bobbin of nonmagnetic material, said armature being of an outer diameter such thatthe armature is slidably received within said central bore, with aminimum clearance, the outer diameter of said armature being between 15and 30% of the outer diameter of said coil.
 3. The invention defined inclaim 1 wherein the axial thickness of said first flux washer is between30 and 50 percent of the axial length of said coil.
 4. The inventiondefined in claim 1 wherein said armature is slidably received withinsaid main passage with a radial clearance R₁ from the wall of said mainpassage, and the radial clearance R₂ between said armature and saidcentral bore of said first flux washer is such that R₁ /R₂ is less than0.2.
 5. The invention defined in claim 4 wherein said armature includesmeans defining fluid flow grooves in the outer surface of said armatureextending axially from one end of said armature to the other.
 6. Theinvention defined in claim 1 wherein said armature, flux washers andouter housing member are of a material selected from the groupconsisting of:a. 2.5% silicon iron b. 12L14 c. 0.45% to 0.9% phosphorusiron.